The present invention relates to an optical thermal imaging system, and more particularly to a numerical aperture limiter for an optical thermal imaging system which uses light in the middle and far infra-red regions to produce an image on a detector surface for the purpose of thermal image acquisition.
Devices incorporating thermal imaging systems are widely used for various civilian and military applications. These devices are highly sensitive to thermal noise, especially to the noise generated by radiation from the devices themselves. Reduction of noise generated by the detector is achieved by cooling down the detector to a low temperature.
Another method of reducing noise comprises the provision of a cold shield in front of the detector plane. The cold shield is a mechanical diaphragm which serves as the real pupil of the entire optical system. The cold shield communicates with the detector Dewar via a cylindrical tube having additional diaphragms, which causes the cold shield to assume the same temperature as the detector.
Only those rays from the object space that hit the entrance pupil (the re-imaged cold shield) and are within the field of view of the device as defined by the detector size and the effective focal plane of the optical system, will reach the detector. All rays which do not meet the above conditions will eventually hit the housing of the device or the cold shield itself, but will fail to reach the detector. The cold shield concept thus ensures the reduction of hot-ray noise arriving at the detector from the scene and improves the signal-to-noise ratio of the system.
However, apart from its advantages, the above-mentioned cold shield system suffers from several drawbacks relative to conventional thermal imaging systems without cold shields. First, the cold shield structure itself constitutes a large, additional mass to be cooled, causing the cooler to consume more power than the conventional thermal system. Then, the optical system is much more complex, requiring an additional optical relay system to re-image the cold shield of the detector into the entrance pupil of the system. Also, owing to the close proximity of the cold shield and the detector plane, the F-number (F/#) rapidly increases with the angle of the field of view. Last, but not least, the change of the angle of incidence of the rays with the field of view causes additional degradation of transmission through the interference cold filter, which is usually designed for some discrete optimal angle.
It is thus one of the objects of the present invention to provide a numerical aperture limiter (NAL) which is based on the principle of total internal reflection (TIR) and will transmit to the detector plane only those rays the angle of incidence of which does not exceed the numerical aperture of the selected system.
It is another object of the present invention to provide an NAL which need not be located in the evacuated and cooled internal space of the detector, but which can be attached to the outside of the detector window, thereby saving the energy wasted in cooling the cold shield structure of prior-art systems.
It is yet another object of the present invention to provide a graded series of NALs, attachable to the outside of a detector window, so that one and the same detector-Dewar-cooler unit can be used for various optical thermal systems by simply selecting the appropriate NAL for that series.
It is still another object of the present invention to provide an NAL that can be used with an optical thermal system without the need for an optical relay.
In accordance with the present invention, there is therefore provided a numerical aperture limiter for an optical thermal imaging system, comprising at least one pair of wedge-like prisms made of a transparent material having a known index of refraction, the prisms of said pair being disposed in close mutual proximity and in opposite orientation with the narrow end of one prism adjacent to the wide end of the other prism, the prism surfaces facing one another of said pair of prisms being spaced apart from one another by a distance at least equal to the wavelength of the light used by said imaging system, with the vertex angle of each one of said prisms being a function of the desired numerical aperture and said index of refraction.
The invention further provides a numerical aperture limiter for an optical thermal imaging system, comprising at least one group of three wedge-like prisms made of a transparent material having a known index of refraction, the prisms of said group being disposed in close mutual proximity and in opposite orientation with the narrow ends of the first and third prisms of said group being adjacent to the wide end of the second prism, the prism surfaces facing one another of said group of prisms being spaced apart from one another by a distance at least equal to the wavelength of the light used by said imaging system, with the vertex angle of the first and third prisms of said group being xcex1 and the vertex angle of the second prism of said group being 2xcex1, with xcex1 being a function of the desired numerical aperture and said index of refraction.